This invention generally relates to the use of foam materials for packaging applications, and more particularly, to the use of elastic memory foam for detection of heat damage to the carton contents.
Foam materials have been widely used for thermal insulation and shock absorption in packaging of fragile or heat sensitive contents. In U.S. Pat. No. 3,420,363 to Blickensderfer, methods for forming and using open-celled foam materials having xe2x80x9cthermal memoryxe2x80x9d are disclosed. Foams found to have thermal memory characteristics include those produced from butadiene liquid polymer and suitable amounts of an activator and sulfur monochloride. The foam materials can be foamed, heated to a softened state, compressed and cooled to a densified state of about 20% its original volume in which it can be worked, then subsequently heated to an elevated temperature of about 150xc2x0 F. to 300xc2x0 F. to re-expand the foam to its original volume. In one application, the compressed foam can be laminated or adhered to various packaging substrates or used as panels or inserts in a carton, then re-expanded to form a snug, shock-absorbing cushion for an article packaged in the carton. In another application, the article is wrapped in a sheet of densified foam and inserted in an enclosing carton, then the foam is re-expanded to immobilize and cushion the article in the carton. In yet another application, the densified foam can be incorporated in a structure of reduced volume, to facilitate transportation, then re-expanded to its greater volume at the point of use.
Recently, a new class of open cellular foam materials have been developed for space applications at the Jet Propulsion Laboratory, California Institute of Technology, Pasadena, Calif., in conjunction with Nagoya Rxc3x97D Center, Mitsubishi Heavy Industry, Nagoya, Japan. These foam materials are described in the proceedings of SPIE ""99 International Symposium on Smart Structures and Materials, March 1999, Newport Beach, Calif., in an article entitled xe2x80x9cCold Hibernated Elastic Memory (CHEM) Self-Deployable Structuresxe2x80x9d, by W. Sokolowski, A. Chmielewski, S. Hayashi, and T. Yamada. The foam materials are polyurethane-based thermoplastic polymers with a wide glass transition temperature Tg range. They can be compressed to as little as 5% of their original volume and re-expanded at the point of use. The material""s memory shape function allows repeated shape changes and shape retention. They are proposed for use as low weight, small volume materials in their densified state that are expandable for large, deployable space structures.
While the prior art has shown the use of thermal memory foam materials as insulative and shock-absorbing packaging materials, and for thermal re-expansion to field-deployable larger volume structures, the present invention seeks to use the thermal memory characteristic of the foam materials for other purposes, namely packaging safety applications as an indicator of heat damage or exposure to elevated or unacceptable high temperatures.
In accordance with a first preferred embodiment of the present invention, a method for packaging an article which contains heat-sensitive contents that may be damaged by heat above a predetermined threshold temperature Tt comprises: selecting an open cellular foam material having a thermal memory characteristic at a glass transition temperature Tg which is approximately equal to or greater than the heat-damage temperature threshold of the contents of the article; forming the foam material to an original foamed volume OV; heating the foam material to a temperature greater than or equal to its glass transition temperature Tg; compressing it at this elevated temperature to a selected densified volume DV; cooling the densified foam material to a temperature below its Tg to retain it in the densified state; applying the densified foam material around an article in a carton leaving a free space in the carton which is defined as the quantity (OVxe2x88x92DV) multiplied by a xe2x80x9cvolume factorxe2x80x9d Y. This free space may be between the article and the foam material, or between the carton walls (side, top, bottom) and the foam material, or any combination thereof. When heat is applied to the carton in such a manner that the foam temperature exceeds its Tg, the foam material tends to re-expand to substantially its original volume OV, resulting in a visible or physical indication that the package contents may have been exposed to a temperature above the predetermined threshold temperature.
If the design packaging system is such that volume factor Y is equal to or greater than 1, there will be visible indication of potential heat damage upon inspecting the interior of the package, as the expanded foam will occupy a noticeable percentage of what was originally free space inside the package. When volume factor Y is less than 1, the expanded foam will occupy essentially all of the available free space in the package, and will exert force upon either the package contents, the package walls, or, in most cases, both the contents and the walls. In this case, the package walls will tend to bulge outward, providing rapid visual indication of exposure to elevated temperatures. If the stiffness of the package walls is such that they resist deformation, the expanded foam will produce an inward force on the contents of the package, making the removal of the contents difficult. In all cases and regardless of package wall stiffness, if Y is less than 1, this inward force will be present to some degree and will tend to make removal of package contents difficult without destroying the package.
As used herein, substantially its original volume shall mean a volume that is less than or equal to its original volume.
In accordance with another embodiment of the invention, a method for packaging an article which contains heat-sensitive contents that may be damaged by heat above a predetermined threshold temperature comprises: selecting an open cellular foam material having a thermal memory characteristic at a glass transition temperature Tg which is approximately equal to or greater than the heat-damage temperature threshold of the contents of the article; forming the foam material to an original foamed volume OV; compressing it at an elevated temperature above its glass transition temperature Tg to a selected densified volume DV; cooling the densified foam material to a temperature below its Tg to retain it in the densified state; applying the densified foam material to a heat-damage-indicating panel located on a wall of the carton having an inner surface adjacent the article and an outer surface visible externally of the carton which is rupturable by the re-expanded foam material applied to the heat-damage-indicating panel, such that when a temperature exceeding the glass transition temperature Tg is applied to the carton and the foam material in the heat-damage-indicating panel, the foam material is re-expanded to substantially its original volume OV to rupture the outer surface of the panel and thereby indicate that the contents of the article may be heat-damaged.